For the measurement of impulse voltages it is valuable to reconstruct the input voltage wave on the high voltage side using the measured output voltage wave and the step response of the measuring system.This paper improves the accuracy of numerical correction using the data fitting method to get the equation for the step response and the cubic spline function to get the first and the second derivatives of the output voltage wave. Examples are given

including the comparison between the expctimental and calculated results from a 1.2 MV impulse resistor divider system. The coincidence in these cases is satisfactory.